Coal and oil fired power plants produce flue gas that contains sulfur dioxide. Limestone (calcium carbonate) and lime (calcium hydroxide) are the traditional sources of alkalinity for scrubbing the flue gas from coal fired power plants. The purity of gypsum produced from a limestone-based scrubbing system varies between about 85 to 95 percent, with 90 percent being typical. The gypsum purity from other lime-based scrubbing systems ranges from about 94 to 98 percent. The pH of gypsum produced from a limestone-based scrubbing system is usually between about 6.0 and 8.0, which is a desirable range for producing wallboard. However, the pH of gypsum produced from lime-based scrubbing systems can exceed 8.0, making the gypsum unsuitable for wallboard production without further processing. The particle size distribution of the gypsum produced from limestone-based scrubbing systems is usually a narrow range of about 10 to 70 microns, whereas the gypsum produced from lime-based scrubbing systems is a broad range of about 10 to 200 microns. A narrow range of particle size distribution is desirable for the production of wallboard. Lime-based scrubbing systems typically have a lower initial installation cost and a higher sulfur dioxide removal efficiency. However, the gypsum produced from a lime-based scrubbing process is generally not suitable for use without further processing such as pH adjustment, treatment with chemical modifiers, or grinding and/or screening.
In lieu of a limestone or lime-based scrubbing system, flue gas containing sulfur dioxide may be treated by passing it through a scrubbing unit that utilizes a magnesium-containing scrubbing agent, e.g., a magnesium hydroxide slurry. The magnesium hydroxide reacts with sulfur dioxide to produce magnesium sulfite and magnesium bisulfite. The magnesium hydroxide slurry also reacts with hydrochloric acid in the flue gas, producing magnesium chloride, although there is very little hydrochloric acid in the flue gas compared to sulfur dioxide. The following chemical reactions occur in the scrubbing unit:SO2+Mg(OH)2→MgSO3+H2O  (1)H2O+SO2+MgSO3→Mg(HSO3)2  (2)Mg(HSO3)2+Mg(OH)2→2MgSO3  (3)2HCl+Mg(OH)2→MgCl2+2H2O  (4)
The blowdown from the scrubbing unit contains magnesium sulfite and magnesium bisulfite. Several processes have been developed to convert this blowdown into valuable products that can be collected for sale or further use, namely, gypsum and magnesium hydroxide. Gypsum may be used to make wallboard and as a soil conditioner on large tracts of land in suburban areas as well as agricultural regions. Magnesium hydroxide is a chemical reagent grade material that may be used for pH neutralization in water treatment, and SO3 removal and boiler slag prevention in coal fired power plants. The purity of magnesium hydroxide generally ranges from about 85 to 95 percent, and most magnesium hydroxide that is sold has a purity ranging from about 88 to 92 percent.
One method for treating flue gas containing sulfur dioxide is marketed by Dravo Lime Company as the ThioClear® process. A wet scrubber with a magnesium hydroxide scrubbing agent is utilized to remove sulfur dioxide from the flue gas. The blowdown from the scrubber passes through an oxidation unit where magnesium sulfite and magnesium bisulfite are converted to magnesium sulfate. Bleed from the oxidation unit is reacted with a slaked lime slurry to crystallize gypsum and magnesium hydroxide particles in a regeneration tank. Thus, the process converts magnesium sulfate blowdown to gypsum and magnesium hydroxide using calcium hydroxide in a single chemical reaction.
U.S. Pat. No. 4,874,591 to Jeney discloses a process for the purification of waste gas containing hydrochloric acid and sulfur dioxide. The waste gas is contacted with a magnesium-containing scrubbing agent, e.g., a magnesium hydroxide suspension, to generate reaction products. A calcium-containing reagent, e.g., calcium chloride, is introduced to the reaction products to precipitate gypsum and generate a chloride-containing liquid. The liquid undergoes pyrohydrolysis to produce a stream of magnesium hydroxide, which is recirculated to the scrubber, and a stream of hydrochloric acid, which is recovered for use in other applications.
Japanese Patent No. JP1222524A to Morita discloses a treatment process for waste water containing magnesium sulfate. The wastewater is mixed with calcium chloride to form a gypsum product that is separated out using a filter press. Calcium hydroxide is added to the separated filtrate to form magnesium hydroxide in a reaction tank. The reaction product is separated into magnesium hydroxide and a filtrate by a filter press, and the magnesium hydroxide is used as the adsorbent for wet desulfurization. Part of the filtrate is sent to a water tank containing calcium chloride and part is sent to a reverse osmosis unit. Water containing calcium chloride from the reverse osmosis unit is recycled into the water tank. Magnesium hydroxide from the reverse osmosis unit is used in a slurry tank containing aqueous magnesium hydroxide slurry.
Because gypsum and magnesium hydroxide represent valuable products, there exists a need for new systems that are capable of efficiently converting scrubber blowdown into these substances. While the prior art provides methods for generating these substances using a magnesium-containing scrubbing agent, new methods could improve the efficiency of existing systems, the quality of the gypsum and magnesium hydroxide produced, and the ability to control pH and particle size distribution.